1. Field of the Invention
The present invention relates to a conductive paste, and more particularly to a conductive paste used for interconnecting wiring layers of a ceramic multilayer wiring substrate on which chip components such as an LSI and an IC are mounted.
2. Description of the Related Art
A green sheet stacking method and a thick film printing method have widely been used for producing a ceramic multilayer substrate. These methods respectively have advantages: According to the green sheet stacking method, wiring layers can be formed with high density to provide high density wiring patterns; According to the thick film printing method, its application covers a wide range because of its simplified production process and good production yield.
In the production of the ceramic multilayer substrate, the interconnection of the respective wiring layers is one of the critical techniques. For the interconnection, in general, a via hole is formed in an insulating layer and the via hole (through hole) is filled with a conductive material. These steps are referred to as a via-fill process. In either of the green sheet stacking method or the thick film printing method, when the thickness of the insulating layer is small, the via hole can be filled with a conductive material simultaneously with the formation of a wiring pattern. However, when the thickness of the insulating layer is large, the via-fill process is needed before forming the wiring pattern.
Conventionally, a conductive paste containing, as its main components, a conductive material, inorganic component powders, an organic binder, and a solvent thereof is used for filling the via hole in the via-fill process.
As shown in FIG. 3, in the case where a via hole 2 of a green sheet insulating layer 4 is filled with a conductive paste 5, a solvent contained in the conductive paste 5 penetrates into the green sheet insulating layer 4 as well as evaporates therefrom; as a result, inorganic component powders contained in the conductive paste 5 move to an inner side of the via hole 2. This phenomenon is called leveling. Because of this leveling, a recess 3 is formed in the center of the conductive paste 5. The formation of the recess 3 means a filling defect, which causes an electrical connection failure between wiring layers. Such an electrical connection failure is a fatal defect for a ceramic multilayer substrate.